In flow control systems, it is desirable to utilize a fluid type actuator to drive a flow control element such as a mechanical valve disposed in the flow stream. Fluid actuators are very effective, reliable, and relatively inexpensive compared to the cost of a comparable electrical actuator that would require an electric motor.
Because of basic advancements made in electronics and electrical control systems over the past years combined with the ease in which electronics can be adapted to control systems, one often finds that the control signal to the actuator is in the form of an electric current signal. In order to accommodate fluid actuators in such control systems, the instrumentation industry has provided current to pressure converters, often referred to as I/P transducers. While I/P transducers of the prior art have met with success and are presently used in many control systems, they nevertheless have shortcomings and disadvantages.
Virtually without exception, I/P transducers of the prior art have moving parts such as a voice coil disposed in operative relationship with a permanent magnet or magnets. Problems associated with moving parts within an I/P transducer are many.
First, moving parts having a relatively significant mass that invariably makes conventional I/P transducers susceptible to hysteresis and deadband because of the requirement of moving the mass, which means that the instrument has poor repeatability. Poor repeatability means less accuracy and precision, and this ultimately results in poor control of the system.
Secondly, the response of I/P transducers of the prior art with moving parts is susceptible and greatly affected by vibration, shock, and change in orientation or attitude. Because the elements of the I/P transducer that produce the output pressure signal are moving parts, vibration, shock or change in attitude or orientation will result in these elements moving. Consequently, the response in situations involving vibration, shock, change in attitude or orientation is not accurate and precise. Again the net result is that the I/P transducer does not accurately and precisely convert the current signal to a correct proportional pressure signal and, there is error in the final control.
Besides the problems associated with the moving parts, most conventional I/P transducers include permanent magnets. These permanent magnets are the source of an additional shortcoming of conventional I/P transducers. Over a period of time, the permanent magnet or magnets experience a degradation in strength that, of course, directly affects the accuracy and precision of the instrument.
Further, most conventional I/P transducers require some type of damping medium. In this regard, some conventional I/P transducers, for example, require oil as the damping medium. This obviously requires the I/P transducer to require maintenance and service.
Finally, I/P transducers of the prior art are big, bulky and often relatively expensive. The size and mass of the I/P transducer is an important consideration since they most often are required to fit in existing panel designs where space is often minimal.